Recently, a method for converting a radiation image to a visible image utilizing a stimulable phosphor has been put in practical use.
In such the method, the radiation image conversion panel comprising a stimulable phosphor layer formed on a substrate such as that described in U.S. Pat. No. 3,859,527 and Japanese Patent Application Publication No., hereinafter referred to as JP-A 55-12144 is used. The stimulable phosphor layer of the radiation image conversion panel is exposed to radiation passed through an object so as to form a latent image or an accumulated image by accumulation of radiation energy corresponding to the radiation permeability of each portions of the object. And then the stimulable phosphor layer is scanned by a stimulating ray using a laser light beam so as to convert and emit the accumulated energy as light (stimulated light) and the intensity of the stimulated light emission is lead to obtain an image. Thus obtained image may be reproduced on various displays such as a CRT or printed out as a hard copy.
High radiation absorbability and light conversion efficiency, excellent graininess and high sharpness are required to the stimulable phosphor layer of the radiation image conversion panel.
Usually, the thickness of the stimulable phosphor layer should be made thicker for raising the sensitivity for radiation. However, there is certain limitation since the light cannot be output by the scatter of the stimulated light between the stimulable phosphor particles.
When the layer thickness is too thin, deterioration on the sensitivity is caused even though the sharpness is raised accompanied with reducing of the layer thickness.
The graininess is depended on the local flicker of quantum number of the radiation or the quantum mottle, and the irregularity of the structure of the stimulable phosphor layer or the structure mottle in the radiation image conversion panel. Accordingly, the reducing of the thickness of the stimulable phosphor layer causes degradation of image quality since the quantum number of the radiation is reduced by the absorption by the stimulable phosphor layer so as to increase the mottle, or the structural irregularity is actualized and the structural mottle is increased. Therefore, a thick stimulable phosphor layer is usually necessary for improving the graininess.
The image quality and the sensitivity of the radiation image conversion method depend on various factors as mentioned above. Until now, various investigations have been carried out for improving the sensitivity and the image quality by controlling the plural factors relating to the sensitivity and the image quality.
Among these methods, it has been tried as the means for improving the sharpness of the radiation image that the shape of the stimulable phosphor is controlled to improve the sensitivity and the sharpness.
A trial of such the methods, for example, JP-A 61-142497 disclose a method in which a stimulable phosphor layer constituted by a pseudo-columnar block which is formed by a sedimentation of the stimulable phosphor on a substrate having a surface with rough pattern.
JP-A 61-142500 describes a method using a radiation image conversion panel having a stimulable phosphor layer in which a crack between the columnar blocks formed by the sedimentation of the stimulable phosphor on the substrate having a fine pattern was subjected to a shock treatment to grow the crack. Furthermore, JP-A 62-39737 describes a method using a radiation image conversion panel in which a crack is formed from the surface side of a stimulable phosphor layer formed on a substrate to make a columnar shape. Moreover, JP-A 62-110200 proposes a method in which a stimulable phosphor layer having a void is formed on the substance by vapor deposition and subjected to a heating treatment to form a crack by growing the void.
Recently, JP-A 62-157600 discloses a method in which when a stimulable phosphor layer is formed on a substrate by vapor-phase deposition method, the crossing angle of a stream line of the vapor of the stimulable phosphor to the substrate surface is controlled within a specified range to form the stimulable phosphor layer having a prescribed thickness; and Japanese Patent No. 2899812 proposes a radiation image conversion panel having a stimulable phosphor layer in which a slender columnar crystallite is formed so that the crystallite makes a certain angle with the normal line direction of the substrate.
In these trials for controlling the shape of phosphor layer, it is object to improve the image quality by making the phosphor layer to the columnar structure. The sharpness of the image of stimulated light emission can be considerably increased by making the columnar structure since the diffusion of the stimulating rays or the stimulated light emission in the horizontal direction since the light reaches to the substrate surface by repeating reflection at the cracks (interface of the columnar crystallites.) In the radiation image conversion panels prepared by the vapor phase deposition method as above-mentioned, a glass plate or a ceramic plate is usually used for providing the heat resistively on the occasion of the vapor-phase deposition, and glass is used as the protective layer for holding the moisture proof ability.
However, the usual radiation image conversion panel in which both of the substrate and the protective layer are constituted by the glass plate or the ceramic plate, each of which does not have a flexibility, has a problem such as that the panel is high in the cost and easily breakable and is as heavy as difficultly handling even though the moisture resistance is excellent. Moreover, further improvement on the image quality is required also to the radiation image conversion panel having the stimulable phosphor layer formed by vapor-phase deposition.